1. Field of the Invention
Embodiments of the invention relate to an organic light emitting display and more particularly to a method for sensing degradation of an organic element of an organic light emitting display.
2. Discussion of the Related Art
An active matrix organic light emitting display includes an organic light emitting diode (hereinafter, referred to as “organic element”) capable of emitting light by itself and has advantages of a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, and the like.
The organic element serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML and form excitons. As a result, the emission layer EML generates visible light.
The organic light emitting display arranges subpixels including the organic element in a matrix form and adjusts a luminance of the subpixels depending on grayscale of video data. Each subpixel includes a driving thin film transistor (TFT), which controls a driving current flowing in the organic element depending on a gate-to-source voltage Vgs between a gate electrode and a source electrode of the driving TFT. A display grayscale (i.e., a display luminance) is adjusted by a light emission amount of the organic element that is proportional to a magnitude of the driving current.
The organic element generally has a degradation characteristic of an increase in an operating point voltage (i.e., a threshold voltage) of the organic element and a reduction in an emission efficiency as an emission time of the organic element passes. Because an accumulated value of currents applied to the organic element of each subpixel is proportional to an accumulated value of gray levels represented in each subpixel, the organic elements of the subpixels may have different degradation degrees. A degradation deviation between the organic elements of the subpixels results in a luminance deviation, and an image sticking phenomenon may be generated by an increase in the luminance deviation.
A related art compensation method for sensing the degradation of the organic element and modulating video data based on a sensing value using an external circuit is known to compensate for the degradation deviation of the organic element. The related art compensation method connects a current source to each subpixel through a sensing line and applies a sensing current from the current source to the organic element. Then, the related art compensation method decides a degradation degree of the organic element based on an anode voltage of the organic element sensed through the sensing line.
However, the related art compensation method has the following problems.
Firstly, the sensing current applied to each organic element has to be uniformly set, so as to accurately sense the degradation of the organic element. For this, the current sources have to be respectively connected to the sensing lines. In this instance, because the number of necessary current sources increases, the manufacturing cost and a circuit design area of the organic light emitting display increase. Furthermore, it is very difficult to uniformly set the sensing currents applied from all of the current sources, and thus it is very difficult to increase the sensing accuracy.
Secondly, the sensing lines may be formed by an independent sensing line structure or a shared sensing line structure depending on a connection structure.
In the independent sensing line structure, the plurality of subpixels disposed on the same horizontal line may be respectively connected to the plurality of sensing lines. Hence, the organic elements may be individually operated, and the degradation degree of each organic element may be directly sensed. However, because one sensing line is assigned to each subpixel, an aperture ratio decreases. Hence, a current density of the organic element increases during when driving the organic element. As a result, a degradation speed of the organic element in the related art organic light emitting display having the independent sensing line structure increases, and life span of the related art organic light emitting display decreases.
In the shared sensing line structure, a plurality of unit pixels disposed on the same horizontal line may be respectively connected to the plurality of sensing lines, and subpixels constituting each unit pixel may share the same sensing line with one another. In the related art organic light emitting display having the shared sensing line structure, because the organic elements cannot individually operate during the degradation sensing (namely, because the organic elements of each unit pixel simultaneously operate), the degradation degree of each organic element cannot be accurately sensed.